1. Field of the Invention
This invention pertains in general to switches that employ contacts maintained in hermetically sealed housings that contain a medium having a higher dielectric strength than air and more particularly to such switches that employ a single drive rod to move the contacts relative to each other.
2. Background of the Invention
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads, short circuits, and low level voltage conditions. Typically, circuit breakers include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors on an electrical system in response to abnormal conditions. In particular, vacuum circuit interrupter apparatus has been known which includes separable main contacts disposed within an insulating housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor which is interconnected with the circuit to be controlled by the circuit interrupter. The other contact is moveable. In the case of a vacuum circuit interrupter, the moveable contact assembly usually comprises a stem of circular cross section having the contact at one end enclosed within the vacuum chamber and a driving mechanism at the other end which is external to the vacuum chamber. An operating rod assembly is provided which carries a rotatable contact bell crank which is slidable on the operating rod and rotates about a pivot pin upon motion of the operating rod. This plate is connected to the stem or drive rod of the moveable contact. Motion of the plate causes motion of the moveable contact into or out of engagement with the fixed contact.
The operating rod assembly is operably connected to a latchable operating mechanism which is responsive to current. When an abnormal condition is reached, the latchable operating mechanism becomes unlatched which causes the operating rod to move to the open position. The motion of the operating rod, in turn, causes the contact or crank to rotate and, as discussed above, this controls motion of the moveable contact. The operation of this type of vacuum interrupter is more fully described in U.S. Pat. No. 5,095,293, issued Mar. 10, 1992 to the assignee of the present invention.
Vacuum interrupters or reclosers are typically used, for instance, to reliably interrupt medium to high voltage AC currents of several thousands of amperes or more. Reclosers are interrupters that are designed to automatically reclose after a given delay and for a given number of times in an attempt to automatically re-establish the circuit. However, from the standpoint of this invention, the principles discussed herein apply equally well to both.
Typically, in accordance with the prior art practices, one vacuum interrupter is provided for each phase of a multiphase circuit and the interrupters for the several phases are actuated simultaneously by a common latchable operating mechanism. This does not permit point-on-wave switching among the multiple phases and can give rise to arcing and uneven contact wear. Co-pending patent application, Ser. No. 09/476,243 (Attorney Docket No. 97-PDC-524), filed concurrently herewith and assigned to the assignee of this invention, addresses this problem by supplying independent means for actuating the vacuum interrupters respectively associated with each phase of the circuit.
In general, the prior art had actuated the circuit interrupters to the open condition before servicing. While that action opens the circuit it does not drain the circuit of residual charge that can, under certain circumstances, create a safety issue. The aforecited co-pending application recognizes the desirability of providing a mechanism for grounding the circuit before maintenance takes place. Accordingly, an optimum design for achieving that objective is desired.